2018-10-12_REVISION - C1982056 (2)

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Introduction 1.1 Project Background The Foidel Creek Mine sits on 20,000 acres in the Uinta Coal Basin. Current coal reserve estimates are approximately 52 million tons of recoverable coal. The mining operation process includes mining coal seams and the associated overburden and interburden units which produce a limited amount of groundwater and drain to the mine workings. Excess water from mine drainage and dust control activities drain to estab3Ished sumps wh'Ich are then pumped to sealed and abandoned mine areas, acting as large underground water storage pools. TC has recently developed rock slopes to provide access for future development and mining of a new coal seam. Two underground pools of accumulated ground water containing approximately 400 million gallons of water will need to be drained for safety reasons. TC is intending to pump the water to the surface and discharge it to a local stream. The pumping operation is estimated to require nine years, with pumping rates varying depending on the volume of stored water at different locations w$thin the mine. The ground water to be discharged includes concentrations of iron (Fe), sulfate (SO4), and total dissolved solids (TDS) concentrations that exceed typical surface water values. To enable surface discharge, the groundwater will require treatment Final discharge criteria are anticipated to be defined through discussions with the Colorado Department of Public Health Safety and Environment (CDPHE). Given the relatively short duraVon of the pumping operation (dewatering), passive treatment systems are the preferred method to be used to improve the water quality of the effluent discharge. Pass}ve treatment systems such as the one covered in this design, require less energy and operational effort to maintain than conventional mechanical or chemical treatment systems, but typically require extensive areas of land to support the level of treatment necessary to meet discharge criteria. The proposed site has adequate area for this application, and has enough elevation change across the site to support a pass',ve flow by gravity through the system. After the completion of the dewatering operation, the wetland system will remain in place as a wildlife habitat and for polishing of site stormwater runoff. Passive treatment systems rely upon naturally occurring physical, chemical and biological processes to improve water quality without active introduction of energy and materials_ Iron is typically removed through precipitation as iron hydroxides and oxyhydroxides in open water where oxygen is present (i.e., aerobic), or complexed as a sulfide compound (e.g., pyrite) in water where oxygen is low or absent (i.e., anaerobic). Sulfate is typically reduced in the same anaerobic environment to sulfide, which then may complex with available iron. Passive treatment of sulfate centers on reducing sulfate to sulfide under anaerobic conditions Total dissolved solids (TDS) represents the total mass of elements and compounds present in the water. The dissolved solids are typically dominated by positively charged elements (i.e., cations) such as calcium, magnesium, sodium and potassium, among other trace metals, and anions that are typically dominated in freshwater by sulfate, carbonate, and nitrate. As such, the reduction of TDS typically occurs in passive treatment systems through sorption of salts to organic and metal surfaces and biological reduction of anions under anaerobic conditions. Of the three compounds of concern, iron has been demonstrated to be readily removed under both aerobic and anaerobic conditions. Of the three compounds comprising treatment criteria for the Foidel Creek mine discharge, iron is the parameter most responsive to passive treatment and serves as the primary basis of sizing in this analysis. PEABOOY FINAL DE51GN TM 20131010 COPYRIGHT 2012 BY CH2M HILL INC • COMPANY CONFIDENTIAL